The idea that plants may take up organic nitrogen compounds such as amino acids dates back to the 19th century. Working at the Rothamsted Experimental Station, Hutchinson and Miller, in this paper, provide an extensive literature review on the uptake of inorganic and organic nitrogen forms by plants.
They show that for the majority of organic compounds, no evidence of uptake had been found, but they also list 15 compounds for which a nitrogen uptake of at least 1 mg had been shown, including urea. They go on to study many of those in a series of solution culture experiments with peas, which also illustrate some of the difficulties associated with this type of research. For example, is it crucial to keep things sterile and avoid contamination with inorganic nitrogen.
Based on the results, they provide a provisional grouping of compounds as (i) readily assimilated, (ii) assimilated, (iii) doubtful and (iv) toxic. Within the first group, the best results were obtained with urea and barbituric acid, the former providing more nitrogen than the latter whilst barbituric acid gave a greater amount of dry matter. Of course, none of this challenges the fundamental quantitative importance of mineral forms of nitrogen for crop nutrition.
